6V6 Class A biasing

[Paul P]

I'm about to bias my amp and have some questions regarding the differences between 'recommended' values and those found in Fender schematics. Particularly the different versions of the Champ using one 6v6 (or two).

Is it possible that biasing in Fender Champs was not ideal with respect to biasing a 6V6 for Class A operation as recommended in the tube specifications ?

Up until now, I've been blindly following Fender's schematics with respect to plate voltages and component values. But reading about Class A biasing, it looks like the plate voltage should be a lot less or the cathode resistor (when present) a lot higher to bring things into line with recommendations found in tube spec sheets.

I've also come to the conclusion that it's not possible to properly bias a 6v6 with fixed bias when the plate voltage is close to 400v, and yet the Super Champ does exactly this (on each 6V6).

My present dilemma is that I thought I could switch from cathode bias to fixed bias on my power tube(s) by simply replacing the cathode resistor/cap bias with a biasing grid voltage, selected by a DPDT switch.



But I now realize that it's not so simple. Cathode biasing reduces the voltage the tube sees compared to fixed bias (somewhere between 25v and 40v). So if I adjust plate voltage and the cathode resistor and get things in spec for proper Class A biasing, I won't be able to adjust fixed bias to the higher voltage the tube sees without the cathode resistor and still be in spec.

The question all this comes down to is this : would it be better to stray from ideal by going lower with the plate voltage and the chosen cathode resistor to then also be able to properly bias with fixed bias, or should I make things ideal for cathode bias and live with whatever departure from ideal is brought on by having too high a voltage across the tube (and adjusting the fixed bias to at least keep dissipation in bounds) ? Or should I just forget about switching between cathode and fixed bias ?

[vintagekiki]

6V6 Bias Chart
How To Bias Power Tubes
1)
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/8554-about-biasing-tube-guitar-amplifiers?t=8291
About biasing tube guitar amplifiers

[Leo_Gnardo]

should I just forget about switching between cathode and fixed bias ?

I presume you have tried the circuit in fixed bias form, had a play, maybe multiple plays on it? How do it sound? To you??? If there's some perceptible sonic advantage then sure, go for it. If not let the Champ be a Champ, self-biased and all. It's nice to see some folks recommend on these pages, it's OK to increase the cathode resistor to tame bias. So we're not stuck with that 470 ohm Rk that once was regarded as sacrosanct. Select one that works for your amp, your 6V6 and your ears.

FWIW I did a number of listening tests @ 25 years ago on mid 60's Ampegs. Some had been converted to run either self-biased or fixed bias, decided by a switch. Some amps ran 7591, others 6V6. I could not distinguish the two sonically, so I stopped offering the mod.

[Helmholtz]

I've also come to the conclusion that it's not possible to properly bias a 6v6 with fixed bias when the plate voltage is close to 400v, and yet the Super Champ does exactly this (on each 6V6).

The Fender Super Champ is a push-pull class AB amp, not comparable to your SE class A amp.

Fixed bias class A operation is not advisable as there is considerable variation of the grid bias to plate current relation between individual tubes.
So bias adjustment would be required over tube lifetime and when replacing tubes.

Cathode bias OTOH employs DC current NFB and self-stabilizes the operating point against variations in supply voltage and tube parameters, so no re-biasing required.

And yes, Leo Fender liked to stretch tube limits in order to squeeze out more power.

[Paul P]

The Fender Super Champ is a push-pull class AB amp, not comparable to your SE class A amp.

You're right of course. I've been reading too many Class A articles and imagined a push-pull Class A (with fixed bias).

FWIW I did a number of listening tests @ 25 years ago on mid 60's Ampegs. Some had been converted to run either self-biased or fixed bias, decided by a switch. Some amps ran 7591, others 6V6. I could not distinguish the two sonically, so I stopped offering the mod.

This and what Helmholtz says (though it's pretty easy to readjust the bias) makes it easier for me to just forget about the idea.

The fixed bias supply is/was there to permit converting the amp to push-pull AB with just the addition of a phase inverter board and swapping the OT to a center-tapped one.

[Enzo]

And remember, this is just a guitar amp, not a super hi-fi circuit. It is a Champ, and works well as a Champ. Is a Champ "ideal"? Maybe not, but it isn't trying to be, it is trying to be a Champ. The whole idea of "properly" biasing the thing rubs me wrong.

[vintagekiki]

https://robrobinette.com/5e3_Modifications.htm#Switchable_Bias
Switchable Bias: Fixed Bias / Cathode Bias

1)
https://www.tdpri.com/attachments/mbse_amp1-jpg.667773/

[pdf64]

Consider using additional resistor/s to drop thd HT supply voltage to the OT (anode), screen grid, or both.
By that, a centre-ish bias should be feasible.

[Paul P]

https://robrobinette.com/5e3_Modifications.htm#Switchable_Bias
Switchable Bias: Fixed Bias / Cathode Bias

If I understand things correctly, it is easier to have switchable bias on a class AB, push-pull output stage (like above) than it is to do the same with a class A, single ended, at least for high plate voltages (higher 300's and above).

With class A, switching to fixed bias adds the voltage dropped across the cathode resistor to the plate and if plate voltage is already high, it'll be difficult to keep things in range (at least to tube specs for class A).

EDIT ==> I've just discovered that I went down a wrong track when I tried to mix Champs and push-pull Fenders. I somehow lost sight of the fact that original Champs had much lower B+ than the push-pull amps.

I'm going to chart all voltages and currents in the amp with different rectifier tubes (5y3GT, 5U4GB, (GZ34)) and then re-learn to compute load lines, operating points etc. (why is that so hard on my head?).

Being visual, I'd really like some way to see class A in operation, to ensure clipping is symetrical and levels in bounds. I've searched the net for examples of using a scope and other test equipment to adjust class A operation, but have found nothing (there's lots for class AB). The alternative appears to be doing some measurements and some math and then hoping that things are actually behaving like they're supposed to.

Is a Champ "ideal"? Maybe not, but it isn't trying to be, it is trying to be a Champ. The whole idea of "properly" biasing the thing rubs me wrong.

I understand, but my greatest difficulty is that I have no idea what a Champ or other vintage amp is 'supposed' to sound like, having never played one. So I follow Fender's schematics and hope for the best. My insecurity comes from not being able to also rely on tube specifications and recommendations since Fender didn't limit themselves to them.

[Helmholtz]

My insecurity comes from not being able to also rely on tube specifications and recommendations since Fender didn't limit themselves to them.

You're not alone.

But fact is that Leo's circuits work, stood the test of time and typically sound great.

I'm not sure that center biasing and symmetrical clipping is desirable in a guitar amp.

[Enzo]

I'm not sure that center biasing and symmetrical clipping is desirable in a guitar amp.

Agree. This is not a hifi.

Never played a CHamp? Chances are you never played any amp you have not yet built. Chances are any copy you build still won't sound just like the original. I'd say just build it and see what it does. It would not be hard to change the cathode resistor later if you want to bias it different. You may not know what a Champ "ought" to sound like, but by the same token, we don't then know what we ought to do to our creation to make it more Champ-ish.

You looked at the power stage clipping symmetrically but what about the preamp stage or stages? Who says they clip evenly? Usually don't.

[pdf64]

A benefit of centre bias is that it should result in the most power output.
Single ended stages, even if they clip symmetrically, will probably have non symmetrical clipping characteristics, due to different mechanisms acting on the top and the bottom.

[Helmholtz]

A benefit of centre bias is that it should result in the most power output.

But not if you need to lower B+ to achieve center bias (post #8) without increasing the idle current.

[Dave H]

But I now realize that it's not so simple. Cathode biasing reduces the voltage the tube sees compared to fixed bias (somewhere between 25v and 40v). So if I adjust plate voltage and the cathode resistor and get things in spec for proper Class A biasing, I won't be able to adjust fixed bias to the higher voltage the tube sees without the cathode resistor and still be in spec.

You could do it like this. To make it switchabe to cathode bias switch the zener to a resistor.
If it were my amp I'd just stick with cathode bias.